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SIGNAL(3C)							    SIGNAL(3C)

NAME
       signal - simplified software signal facilities

SYNOPSIS
       #include <signal.h>

       (*signal(int sig, void (*func)(int)))(int);

DESCRIPTION
       Signal  is  a  simplified  interface  to	 the  more  general  sigvec(2)
       facility.

       A signal is generated by some abnormal event, initiated by a user at  a
       terminal (quit, interrupt, stop), by a program error (bus error, etc.),
       by request of another program (kill), or	 when  a  process  is  stopped
       because	it  wishes  to	access	its  control  terminal	while  in  the
       background (see tty(4)).	  Signals  are	optionally  generated  when  a
       process resumes after being stopped, when the status of child processes
       changes, or when input is ready at the control terminal.	 Most  signals
       cause  termination of the receiving process if no action is taken; some
       signals instead cause the process receiving them to be stopped, or  are
       simply  discarded  if  the process has not requested otherwise.	Except
       for the SIGKILL and SIGSTOP signals, the	 signal	 call  allows  signals
       either  to be ignored or to cause an interrupt to a specified location.
       The following is a list of all signals with names  as  in  the  include
       file  <sys/signal.h>.   These  are  passed  to  the  signal call in the
       argument sig.

       SIGHUP	 1    hangup
       SIGINT	 2    interrupt
       SIGQUIT	 3*   quit
       SIGILL	 4*   illegal instruction
       SIGTRAP	 5*   trace trap
       SIGIOT	 6*   IOT instruction
       SIGEMT	 7*   EMT instruction
       SIGFPE	 8*   floating point exception
       SIGKILL	 9    kill (cannot be caught or ignored)
       SIGBUS	 10*  bus error
       SIGSEGV	 11*  segmentation violation
       SIGSYS	 12*  bad argument to system call
       SIGPIPE	 13   write on a pipe with no one to read it
       SIGALRM	 14   alarm clock
       SIGTERM	 15   software termination signal
       SIGURG	 16@  urgent condition present on socket
       SIGSTOP	 17|+'stop (cannot be caught or ignored)
       SIGTSTP	 18|+'stop signal generated from keyboard
       SIGCONT	 19@  continue after stop
       SIGCHLD	 20@  child status has changed
       SIGTTIN	 21|+'background read attempted from control terminal
       SIGTTOU	 22|+'background write attempted to control terminal
       SIGIO	 23@  i/o is possible on a descriptor (see fcntl(2))
       SIGXCPU	 24   cpu time limit exceeded (see setrlimit(2))
       SIGXFSZ	 25   file size limit exceeded (see setrlimit(2))
       SIGVTALRM 26   virtual time alarm (see setitimer(2))
       SIGPROF	 27   profiling timer alarm (see setitimer(2))
       SIGWINCH	 28@  Window size change
       SIGUSR1	 30   User defined signal 1
       SIGUSR2	 31   User defined signal 2

       The starred signals in the list above cause a core image if not	caught
       or ignored.

       If  func	 is  SIG_DFL, the default action for signal sig is reinstated;
       this default is termination (with a core	 image	for  starred  signals)
       except  for  signals  marked  with @ or |+'.  Signals marked with @ are
       discarded if the action is SIG_DFL; signals marked with |+'  cause  the
       process to stop.	 If func is SIG_IGN the signal is subsequently ignored
       and pending instances of the signal are discarded.  Otherwise, when the
       signal  occurs  further	occurrences  of	 the  signal are automatically
       blocked and func is called.

       A return from the function unblocks the handled	signal	and  continues
       the  process  at	 the point it was interrupted.	Unlike previous signal
       facilities, the handler func remains installed after a signal has  been
       delivered.

       If a caught signal occurs during certain system calls, causing the call
       to terminate prematurely, the  call  is	automatically  restarted.   In
       particular  this	 can  occur during a read or write(2) on a slow device
       (such as a terminal; but not a file) and during a wait(2).

       The value of signal is the previous (or initial) value of func for  the
       particular signal.

       After  a fork(2) or vfork(2) the child inherits all signals.  Execve(2)
       resets all caught signals to the default action; ignored signals remain
       ignored.

RETURN VALUE
       The previous action is returned on a successful call.  Otherwise, -1 is
       returned and errno is set to indicate the error.

ERRORS
       Signal will fail and no action will take place if one of the  following
       occur:

       [EINVAL]	      Sig is not a valid signal number.

       [EINVAL]	      An  attempt  is  made  to ignore or supply a handler for
		      SIGKILL or SIGSTOP.

       [EINVAL]	      An attempt is made to ignore SIGCONT (by default SIGCONT
		      is ignored).

SEE ALSO
       kill(1),	 ptrace(2),  kill(2),  sigvec(2),  sigblock(2), sigsetmask(2),
       sigpause(2), sigstack(2), setjmp(3), tty(4)

4th Berkeley Distribution      January 26, 1989			    SIGNAL(3C)
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